The present invention relates generally to systems and methods for protecting a turbocharger against a shaft overspeed condition. More specifically, the invention concerns using engine fueling commands to prevent turbocharger overspeed.
Internal combustion engines having turbocharger units attached thereto are commonplace in the automotive, heavy-duty truck and industrial vehicle industries. Such turbocharger units are generally responsive to at least some of the engine exhaust gas to increase air pressure in an intake manifold (i.e., boost pressure) and correspondingly increase engine performance.
One engine turbocharger system 10 is depicted in FIG. 1. The system 10 includes an air intake manifold 11 supplying air to each engine cylinder C. The gaseous products of combustion are discharged through an exhaust manifold 12. The engine includes a control computer or engine control module (ECM) 15 that receives signals 16 from various sensors throughout the engine and vehicle. The ECM 15 includes software routines that use these signals to control engine functional components.
The system 10 further includes a turbocharger 20 associated with the engine and coupled between the intake manifold 11 and the exhaust manifold 12. The turbocharger 20 can be of known construction in which fresh air passes through an inlet 21 to a compressor 22 that provides a compressed charge of air to the intake manifold 11. The compressor 22 is connected to a turbine 24 by a turbocharger shaft 25. Exhaust gas is fed through the exhaust manifold 12 to drive the turbine 24 for eventual discharge through outlet 26.
It is known that for normal turbocharger operation the speed of the turbocharger is primarily a function of the engine speed, load and the ambient air pressure. Increased engine power increases the mass flow rate of exhaust gas through the manifold 12, which causes the turbine 24 to rotate faster. The turbocharger shaft 25 directly links the compressor 22 to the turbine 24 so the compressor speed also increases, leading to greater compression of the incoming charge. The compressed air charge is fed to the engine cylinder C through the intake manifold 11. The increase in pressure of the ambient air produced by the compressor 22 is the boost pressure of the turbocharger. The pressure ratio of the turbocharger is the ratio of the boost pressure to the ambient pressure of the air supplied at the turbocharger inlet 21.
Under certain operating conditions, it is desirable to limit the rotational speed of the turbocharger 20. Thus, the system 10 can include a bypass conduit 30 that is connected between the engine exhaust manifold 12 and the turbocharger exhaust outlet 26, essentially bypassing the turbine 24. A variable opening valve 31 controls the amount of exhaust gas passing from manifold 12 to the bypass conduit 30. When the valve 31 is closed, all engine exhaust is supplied to the turbine. When the valve is fully open, exhaust gas is fed to the turbine and to the bypass conduit in proportion to the flow areas of the two paths.
In one type of turbocharger system, the wastegate valve 31 is connected to an actuator 33 by a linkage 34. The actuator 33 is fluidly connected to the intake manifold 11 by an actuator conduit 36 and electrically connected to the engine controller 15 by signal line 35. The wastegate actuator 33 can be of the type shown and described in U.S. Pat. No. 5,755,101, which description is incorporated herein by reference. As disclosed more fully in the '101 patent, the actuator includes a diaphragm connected to the linkage 34, which is modulated in response to the pressure signal provided through actuator conduit 36. In addition, the actuator 33 can include an electrical coil that is energized or de-energized in response to control signals 35 generated by the ECM 15. The actuator 33, and ultimately the wastegate valve 31, can control the speed and boost pressure of the turbocharger 20.
Excessive turbocharger speed can lead to failures of the shaft 35, or to rupture or dislodgment of turbine or compressor blades. Consequently, the wastegate 30,31 is typically calibrated to open at a predetermined boost pressure on conduit 36 to limit the turbocharger shaft speed independent of overall engine speed. In the event of a wastegate failure, a potential exists for the turbocharger wheelspeed to increase above design limits. Turbocharger overspeed is not only detrimental to the turbocharger unit itself, it can also lead to excessive cylinder pressure that causes progressive damage to the engine. Wastegate failures can occur at any point in the system, such as the linkage 31, actuator 33, pressure conduit 36 or ECM control signal line 35.
As yet, no turbocharger and wastegate system, particularly those used with electronic fuel systems, has been provided that accounts for possible wastegate failures. There is a need for a system and method that can protect a turbocharged engine from damage or failure that has followed wastegate failures.